The Epidemiology Branch is conducting a number of birth defect studies in collaboration with the Health Research Board and Trinity College, Dublin, Ireland. The main objective of these studies is to determine the relationship between folate and birth defects. The birth defects studied to date are neural tube defects (NTDs), oral clefts, congenital heart defects, Down syndrome and omphalocele. These studies focus on biochemical factors in the area of folate metabolism, and on genetic mutations in folate related genes associated with birth defects. In the past we have shown that elevated homocysteine is a risk factor for NTDs, that a mutation in the methylenetetrahydrofolate reductase (MTHFR) gene 677C->T is a risk factor for NTDs, and that a small dose of folic acid (100-200 micrograms) can raise red cell folate to levels that can prevent a fifth to almost a half of NTDs. We have shown that methylenetetrahydrofolate reductase (MTHFD), an important gene in the production of purine and pyrimidine for DNA synthesis is a risk factor for NTDs. Mothers who have the R653Q variant of this gene are at increased risk of having a child with an NTD. [unreadable] [unreadable] This past year, we have expanded our work on MTHFD, confirming in a second population that the MTHFD R653Q variant is a maternal risk factor for NTDs. We have examined other polymorphisms and variants related to folate metabolism as potential NTD risks. The reduced folate carrier is essential for transfer of folate into cells. A variant of the enzyme gene, H27R, has been reported to be a risk factor for NTDs, but this is uncertain. We stuied this variant in a large Irish population and showed that it was unrelated to NTDs in cases, their mothers and their fathers. [unreadable] [unreadable] Both the MTHFR C677T and A1298C variants have been reported to reduce the risk for some cancers. The biochemical effect of the MTHFR C677T variant is clear--it reduces folate stores; however, the biochemical effect of the MTHFR A1298C variant is unclear, perhaps because it is in linkage dysequilibrium with the MTFHR 677C "wild type" SNP. We were able to measure the effect of MTHFR A1298C independently of the MTHFR 677 effect, showing the the A1298C variant produces increased cellular (red cell) folate levels. This is a rare example of SNPs on the same exon producing opposite effects and it suggests that MTHFR 1298C influences cancer risk by a different mechanism than MTHFR 677T. We continue to investigate the role of other genes as NTD risk factors.